metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale); Color: White to light grey with reddish iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None; Specific gravity: 2.6–2.7
- Hardness
- 7 (Mohs scale)
- Color
- White to light grey with reddish iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: White to light grey with reddish iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None; Specific gravity: 2.6–2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. Under intense heat and pressure, quartz grains recrystallize and fuse together. It can range in age from Precambrian to more recent geological eras.
Uses & applications
Used in construction as crushed stone for road grit and railroad ballast. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal. It is also used as a decorative stone and in landscaping.
Geological facts
Quartzite is so tough that it often survives erosion better than the rocks surrounding it, forming prominent ridges or hills called hogbacks. It is harder than steel and will easily scratch glass.
Field identification & locations
Identify in the field by its grainy, 'sugary' texture and extreme hardness. Unlike sandstone, it will not crumble easily, and unlike marble, it will not react to acid. Commonly found in mountainous regions or as river-worn cobbles.
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